Chapter 8.3: Qualitative Analysis

Question 1

How do you test for a carbonate?

Answer 1

Add dilute nitric acid, if effervescence is observed the compound is probably a carbonate as if the gas is bubbled through lime water the lime water will become cloudy. Proving the presence of carbon dioxide.

Question 2

How do you test for a sulphate?

Answer 2

Add barium ions (usually through barium chloride) to a solution. If a precipitate is formed the solution is a sulphate or a carbonate.
*Caution over halide addition

Question 3

How do you test for Halides?

Answer 3

Add silver halides (usually through silver nitrate) to the solution. If a precipitate forms Halides are present.

Question 4

How do you distinguish between halides in a halide test?

Answer 4

Precipitate colour: Chloride (white), Bromide (cream), Iodide (yellow)
Chloride will dissolve in dilute aqueous ammonia
Bromide will dissolve in concentrated aqueous ammonia
Iodide will not dissolve in aqueous ammonia

Question 5

How do you order qualitative tests of an unknown compound?

Answer 5

1. Carbonate tests (no false positives)
2. Sulphate tests (false positive from carbonate)
3. Halide tests (false positive from carbonate and sulphate)

Question 6

How do you do a qualitative analysis of a mixture?

Answer 6

• Carbonate test until complete reaction (nitric acid excess- no later false positives)
• Sulphate test until complete reaction (barium excess then filter out precipitate)
• Halide test

Question 7

How do you identify the ammonium ion in solution?

Answer 7

Add a source of hydroxide ions (NaOH)
Warm solution
Test for ammonia gas exiting solution with moist litmus paper turning blue and then being bleached